1. Field of the Invention
The invention generally relates to substrate processing systems and, more particularly, to a cooling system for a magnetron sputtering apparatus.
2. Description of the Background Art
A physical vapor deposition chamber (also known as a sputtering chamber) comprises a vacuum chamber containing a pedestal for supporting a substrate, a vacuum pump for evacuating the chamber, a sputtering target, a gas source, and a power source. To enhance the sputtering efficiency and improve target utilization, a planar magnetron system is coupled to the sputtering target. The planar magnetron system may comprise a rotating magnetron disposed behind the target, opposite the surface to be sputtered. The chamber uses either a DC bias between the target and the substrate and/or an RF source coupled to an antenna surrounding the space between the target and the substrate to form a plasma in the chamber.
The magnetron comprises a magnet assembly that provides a magnetic field near the sputtering surface of the target. A negative bias voltage between the target and the plasma region accelerates ions toward the target to dislodge target material therefrom. The magnetic field produced by the magnetron confines free electrons, including secondary electrons displaced from the target material, near the target to maximize ionizing collisions of the free electrons with the sputtered material and maximize ionization of the plasma gas that is near the target.
The magnet assembly comprises one or more magnets that rotate around the backside, i.e., the nonsputtered surface, of the target, to evenly distribute the magnetic field across the sputtering surface of the target to result in uniform sputtering of the target material. Typically, a motor assembly mounted to the magnet assembly imparts rotational motion to the magnet assembly by rotating a drive shaft coupled to the magnet assembly.
The above described magnetron sputtering process, including the effect of the bias voltage and the magnetic fields, causes a considerable amount of energy to be dissipated by the target and the magnetron, thereby tending to heat the target and the magnetron. Heating of the magnetron and/or the target above a designated processing temperature may adversely affect performance of the process by changing the sputtering rate or reducing sputtering uniformity of the target. Additionally, excess heat may shorten the useful lives of the magnetron and the target and cause mechanical features of the magnetron to wear out prematurely. Furthermore, excess heat may cause thermal expansion of components within the chamber, which can cause closely spaced components, such as the target and the magnetron, to physically interfere with one another.
To alleviate this problem, the magnetron is typically housed in a cooling cavity. A coolant, such as deionized water or ethylene glycol, is flowed through the cooling cavity to cool the backside of the target and to cool the magnetron. However, it is difficult to cool the central area of the backside of the target and a region of the magnetron about the rotational central axis of the magnetron. Centrifugal force generated by the rotational movement of the magnetron tends to force coolant away from the rotational center of the magnetron and away from the central area of the backside of the target. In addition, typically, the presence of a motor assembly above the target and the magnetron further complicates the design of a coolant delivery system that delivers coolant to the central area of the backside of the target and to a region of the magnetron about the rotational axis of the magnetron.
Therefore, a need exists in the art for an improved cooling system for a magnetron sputtering apparatus.